This invention relates to x-ray imaging apparatus. It relates especially to a detector for detecting x-ray or gamma ray photons that have been emitted from a human organ or other object to produce an indication of the composition of that organ or object. For example, in nuclear medicine applications a radioactive tracer may be placed in a fluid that tends to accumulate in a certain organ in the body. By detecting photons emitted by the radioactive material for a certain period of time, an image of that organ may be obtained, which image may show irregularities or abnormalities in that organ.
In some cases the detector used in such imaging apparatus is simply a sheet of film that is sensitive to incident radiation, with the area of the film being commensurate with the size of the organ or other object being examined.
Film-type detectors, however, have certain drawbacks. In the first place, the radiation sensitive grains in the film (e.g. photosensitive silver compounds) are not very dense. Therefore they are not very efficient as photon absorbers. Therefore the organ or other object must be exposed to radiation for a relatively long period in order to develop a reasonably good image.
Also, for best results in terms of image sharpness and clarity, it is desirable that the radiation to which the organ or object is exposed be mono-energetic and that the individual photons travel in straight lines from the source through the organ to the detector. However, as a practical matter, some of the photons are scattered as they pass through the organ. Consequently, when these photons reach the detector they appear to the detector as having originated at different points in the organ. In other words, the detector does not discriminate between scattered photons and those which have not been scattered by the organ or object. This results in blurring and loss of definition in the image on the film. Further, film must be developed before the image is available thereby delaying diagnosis and, of course, the film is not reusable and requires mounting fixtures to properly support it in a planar state.
Recently there has been developed a radiation detector employing a so-called super-heated, super-conducting colloid (SSC). Basically, the detector comprises a colloid body composed of small grains of a dense super-conducting material suspended in a less dense binder. The body is subjected to a low temperature and an external magnetic field that maintain the grains in a metastable super-conducting state in the absence of radiation. When a grain is impacted by a photon, it undergoes a transition from the metastable super-conducting state to the normal conducting state. This transition produces a magnetic flux change in the region of the grain and the flux change is detected by a sensing coil on the surface of the body that has at least one loop encircling the grain in question. As each grain within that loop changes to its normal conducting state in response to an incident photon, a voltage pulse is developed in the sensing coil reflecting that change. Consequently the number of pulses detected provides an indication of the intensity of the radiation incident on the detector. See French Pat. application No. 7536494.
Until now, however, the so-called SSC detectors have had relatively low quantum detection efficiency and very small cross sectional area. Therefore they have not seen applications in radiology and nuclear medicine. The inefficiency stems from the fact that the usual SSC detector is relatively thin (e.g. 1 mm). Therefore a relatively high percentage of the incident photons do not have an opportunity to interact with, and be absorbed by, the colloid grains in the detector. It is no solution to increase the thickness of the usual SSC detector because a sensing coil in the detector having a width W responds to flux changes produced only by those grains which reside within a distance of approximately W/2 above or below the plane of the coil. Therefore a sensing coil on the surface of the typical colloid body does not detect photon interactions with grains near the center of the body. Likewise, a coil imbedded in the body may not detect events occurring near the surfaces of the body. Thus even though more photons may interact with the detector grains, many of these interactions would not be detected by the sensing coil so that there would be no net gain in detection efficiency. Thus, in order to obtain an image of the object being irradiated, with the usual SSC detectors, a relatively long period of exposure is still required, presenting a potential health hazard in the case of animals and humans.